n8upham
commented
7 months ago Many thanks for outlining these issues here @JelleZijlstra -- super helpful. Sounds like what we need is an MDD-specific set of criteria that we use to justify the biogeographicRealm and continentDistribution assignments. Ideally this would be referenced for different more controviersial boundaries, drawing on outside works and/or explaining our reasoning. We could then post this information on the About page for maximal transparency. How far away are we from that type of explication, based on what you've written above + other references that you've seen (ideally relying on more authoritative sources than Wiki)?
JelleZijlstra
I spent a bit of time recently cleaning up the "continentDistribution" and "biogeographicRealm" columns. I fixed most of the obvious issues, but there's room for more clarification. We probably won't have time soon to dive deep into this area, so I'll just write up some thoughts here.
Definitions
The MDD defines its biogeographic realms based on a map in Wikipedia/Commons: https://commons.wikimedia.org/wiki/File:Ecozones.svg. However, this is of course editable by anyone, and if you look at the history, there have been a few changes recently. For example, someone moved the Sulawesi area from Indomalaya to Australasia and someone else moved Fiji and Vanuatu from Australasia to Oceania. The map is also not supported by any citations. I don't think any part of the map is necessarily wrong, but it would be better for MDD to adopt its own definitions that don't rely on an external map that keeps changing. I also have some nitpicks with a few of the specific boundaries; see below.
MDD already uses its own definitions for the continents, which is good.
Indomalaya/Australasia boundary (realms) and Asia/Oceania boundary (continents)
Our current definitions place the realm boundary between Indomalaya and Australasia at Wallace's Line, meaning that Indomalaya contains the Sunda Shelf and the Philippines, and Australasia contains Sulawesi, the Lesser Sunda Islands, and the Moluccas. However, the continent boundary between Asia and Oceania is at Weber's line, which is between Sulawesi and the Moluccas. Therefore, the Wallacea area (Sulawesi and the Lesser Sundas) is in the continent of Asia but the realm of Australasia:
The two different boundaries are sort of helpful in that they emphasize the taxa that occur in Wallacea, but I'd prefer to make that explicit. For example, we could add Wallacea as a separate biogeographic realm, or add subregion distributions within Indonesia.
Australasia and Oceania
The latest release mostly uses "Australasia/Oceania" as a realm and "Oceania" as a continent. The source map on Wikipedia separates "Australasia" from "Oceania" and the slash feels awkward, so I recently made a change to separate "Australasia" from "Oceania" as separate realms. This restricts Oceania to only the remote islands of the Pacific (e.g., Fiji and Hawaii), with very few native mammals.
Now there's a new problem: the definitions of Oceania the continent and Oceania the biogeographic realm are different, because the continent includes Australia, New Guinea, and nearby islands, but the biogeographic realm does not. To fix this I would recommend to also add Australasia as a continent. If we also adjust the boundary with Asia (above section), "Australasia" would mean the same thing in the continent and realm columns, which is good.
Palearctic/Indomalaya boundary
The Wikipedia map places the boundary between the Palearctic and Indomalayan regions approximately as follows: from western Pakistan through India along the southern margins of the Himalayas, then into southern China, with approximately the Yangtze River as the boundary. The map has a long bulge of Palearctic in China south of the northernmost Indomalayan area. Taiwan contains both Indomalaya (lowlands) and Palearctic (mountains in the east). The Ryukyu Islands appear to be mostly Palearctic but it's hard to see them.
In practice MDD tends to favor putting things in the Palearctic. I moved a lot of South Asian species that live in the lowland parts of India into Indomalaya, but there are also a lot of species from South China that are currently placed in the Palearctic but probably should be in Indomalaya.
I would favor putting Taiwan fully in Indomalaya, since its mammal fauna as far as I can tell is mostly Indomalayan, though there are some Palearctic elements. I don't have strong feelings on exactly where the boundary should be within mainland China, but I don't think the exact boundary in the Wikipedia map is very defensible.
Palearctic/Afrotropic boundary
The Wikipedia map places this boundary first on a straight line through the Sahara (looks like about 20°N), passing through Mauritania, Mali, Algeria, and Niger, then a little further south in Chad, then a little further north at the Egypt/Sudan boundary. The Arabian peninsula is also in the Afrotropic, with the boundary some way further north in a straight line passing through Saudi Arabia. That means Yemen, Oman, the UAE, Bahrain, and Qatar are fully in the Afrotropic. A small area of coastal southern Iran is also in the Afrotropic.
Some consequences:
My thoughts:
Nearctic/Neotropic boundary
Here the Wikipedia map places the boundary in the middle of Mexico, with much of the western and eastern coast in the Neotropic (almost reaching the southern tip of Texas in the east) and the north and the central highlands in the Nearctic. Elsewhere the southern tips of Florida and Baja California are in the Neotropic, as are all Caribbean islands.
I would prefer to put all of Baja and Florida in the Nearctic, both for simplicity and because I don't think either area has a lot of Neotropical elements in the mammal fauna.